FIG. 16 schematically shows electric discharge characteristics of a solid polymer fuel cell to which fuel is supplied from a hydrogen storage alloy. The horizontal axis represents the time lapsed after an electric device starts consuming electric power of the fuel cell. The vertical axis represents an output voltage of this fuel cell. As understood from the electric discharge characteristics of FIG. 16, the fuel cell receives a sufficient amount of hydrogen in an early stage of the discharge.
Although, in the process of consuming electric power, the residual amount of hydrogen gradually decreases, the output voltage remains substantially constant. Then, at the final stage of the discharge where substantially no hydrogen fuel remains, the output voltage suddenly drops down to a very lower level and accordingly the electric device cannot operate normally.
Furthermore, as a notable characteristic, the above output voltage does not substantially depend on the pressure of supplied hydrogen.
Namely, the fuel cell maintains a constant output voltage regardless of reduction in the residual amount of the fuel. In other words, a residual amount detecting system relying on the voltage check of a battery is not applicable to the fuel cell device.
Hence, as a battery check method for the fuel cell device, various residual amount detect methods have been already proposed.
It is, for example, possible to prepare a plurality of fuel tanks and estimate a residual hydrogen amount by employing a valve open/close control and using the fuel tanks in turns (refer to the Japanese Patent Application Laid-open No. 2001-295996).
Furthermore, it is possible to combine a hydrogen storage tank with a residual fuel amount detecting tank and control the flow rate of respective tanks to estimate the residual hydrogen amount (refer to the Japanese Patent Application Laid-open No. 2003-139298).
In another method, the initial capacity and the consumed electric power are used to calculate the residual battery capacity (refer to the Japanese Patent Application Laid-open No. H11-230813, the Japanese Patent Application Laid-open No. 2004-171945, the Japanese Patent Application Laid-open No. 2003-346856, or the Japanese Patent Application Laid-open No. 2003-346857).
Furthermore, it is desirable to indicate a residual capacity of the battery detected according to the above-described battery check methods as precise as possible. Accordingly, it is desirable to employ a multistage display of the residual battery capacity.
Therefore, a stepwise display is proposed using a predetermined pattern efficiently indicating the degree of residual battery capacity (refer to the Japanese Patent Application Laid-open No. H09-152653).
A system calculating the residual amount based on the initial fuel capacity and a used amount cannot automatically detect the initial fuel capacity in a hydrogen storage alloy. Therefore, this system requires presetting of the initial fuel capacity to a constant value, or requires a manual input of the fuel initial capacity.
On the other hand, in the case that the initial fuel capacity is fixed to a constant value, it will be difficult to accurately detect the residual amount if a hydrogen storage alloy having an unpredictable initial capacity is used.
Furthermore, manually inputting the initial capacity is time-consuming and troublesome. In addition, an expensive flow rate meter will be necessary to detect the used fuel amount, or a storage tank for a resultant product may be required. As a result, the device will have a complicated arrangement and downsizing and cost reduction of the device cannot be achieved.